Answer: HA + H2O ⇌ H3O+ + H- (option #4)
Explanation: Since the acid is weak you have to use a ⇌ (equilibrium) sign. Equilibrium is denoted in a chemical equation by the ⇌ symbol. Also, when any acid dissolves into water, it produces hydronium (H3O+ or H+). Therefore, the fourth chemical reaction is your answer.
